A unique property of magnetic resonance imaging (MRI) is the ability to selectively image different chemical species by virtue of what is known as the chemical shift phenomenon. For example, in the human body the fat exhibits a resonance which is separated from the water resonance by approximately 3.5 ppm. At a field strength of 2 Tesla this equates to a frequency separation of about 280 Hz in the NMR spectrum. Silicon exhibits a chemical shift of approximately 5 ppm.
It is common that a "water only" image is required in which the fat or silicon resonance must be either saturated prior to each data acquisition or not excited at all.
Suppression Techniques
Techniques of fat suppression, generally referred to as FATSAT hereinafter, are well known in the prior art. Fat suppression is usually achieved by placing a narrow band spectral suppression pulse before the imaging sequence. This pre-pulse is quickly followed by the imaging sequence so that the fat protons do not have time to relax back to their equilibrium magnetization which remains dispersed (saturated) and unable to contribute signal to the image.
Reference is now made to FIG. 1 which is a schematic graph Illustrating a prior art 3D gradient-echo motion compensated sequence having a frequency selective simple gaussian FATSAT spectral suppression pulse which is centered on the fat frequency. All the horizontal axes of FIG. 1 represent time. The double-headed arrow labeled Tp represents the presaturation sequence and the double-headed arrow labeled TR represents the imaging sequence. The graph labeled RF represents the RF imaging sequence which includes a gaussian spectral suppression pre-pulse 2 followed by an imaging RF pulse 3 having a flip angle .alpha..
The vertical axis (not shown) of the graph labeled RF represents the amplitude of the RF pulses. The graphs labeled Gs, Ge and Gv represent the slice selection gradient, the phase encoding gradient and the viewing gradient sequences, respectively, the vertical axes (not shown) of the graphs labeled Gs, Ge and Gv represent the gradient magnetic field amplitude. The slice selection gradient sequence includes a three-lobed motion compensated gradient 5, a phase encoding gradient 6 and a rewinder gradient 7. The gradient pulse referenced 4 is a spoiler pulse which is part of the presaturation sequence Tp. The phase encoding axis sequence includes a phase encoding gradient 8 and a rewinder gradient 9. The viewing gradient sequence (readout gradient) includes a readout gradient 10.
The graph labeled S is the signal graph. The vertical axis (not shown) of the signal graph represents the RF echo signal amplitude. The signal graph includes the RF echo signal 11.
Reference is now made to FIG. 2 which is a schematic graph representing a prior art NMR spectrum of fat and water protons on which a fat gaussian spectral suppression pulse is superimposed. The horizontal axis represents the chemical shift in parts per million (PPM) units. The curve labeled 14 represents the absorption spectrum of fat and water protons. The arrow labeled W indicates the peak absorption of the water protons at 0 ppm and the arrow labeled F indicates the peak of absorption of the fat protons which is shifted by 3.5 ppm relative to the peak absorption of the water protons. The excitation spectrum of a typical fat gaussian spectral suppression pulse 16 is superimposed on the absorption spectrum curve 14. The gaussian suppression pulse 16 is centered at the peak F and will thus selectively excite the fat protons without substantial excitation of the water protons. Suppression pulses using the Sinc function are also known in the art.
Gaussian and Sinc type suppression pulses are required to be long in duration in order to achieve a suitably narrow spectral selection. At a field strength of 2 Tesla a typical Sinc suppression pulse may take up to 26 ms.
Reference is now made to FIG. 3 which is a schematic graph illustrating a prior art 3D gradient-echo imaging sequence having a frequency selective binomial FATSAT spectral suppression presaturation pulse which is centered on the water frequency.
The fat suppression sequence of FIG. 3 uses a 1-3-3-1 binomial suppression pulse which is centered around the water frequency.
The use of binomial pulse suppression techniques is disclosed in an article appearing in The Journal of Magnetic Resonance, entitled "Solvent Suppression in Fourier Transform Nuclear Magnetic Resonance" by P. J. Hore (Vol. 55, 1983, pp. 283-300) incorporated herein by reference.
It is noted that for a better understanding, like components are designated by like reference numerals throughout the various figures.
The gradient-echo sequence of FIG. 3 is similar to the gradient-echo sequence of FIG. 1, except that the RF imaging sequence which includes a gaussian spectral suppression pre-pulse 2 of FIG. 3 includes a 1-3-3-1 binomial suppression pulse 1 instead of the gaussian spectral suppression pre-pulse 2 of FIG. 1. The 1-3-3-1 binomial suppression pulse 1 includes four sub-pulses 1A, 1B, 1C and 1D which are separated from each other by a pulse separation interval .tau.. Similar to the graphs of FIG. 1, the horizontal axes of the graphs of FIG. 3 represent time, the vertical axes (not shown) of the graphs labeled, Gs, Ge, Gv represent gradient magnetic field amplitude, the vertical axis (not shown) of the graph labeled RF represents the amplitude of the RF pulses and the vertical axis (not shown) of the graph labeled S represents the RF echo signal amplitude.
By choosing the appropriate pulse separation interval .tau. (dependent upon field strength and chemical species) the binomial pulse 1 exhibits a null excitation at the water frequency which rises to a 90.degree. excitation at the fat frequency.
Reference is now made to FIG. 4 which is a schematic graph illustrating the theoretical excitation spectrum of a prior art 1-3-3-1 binomial suppression pulse as a function of frequency offset from the transmitter frequency. The vertical axis of the graph represents the transverse magnetization M.sub.xy wherein full scale corresponds to complete conversion of Z magnetization into XY magnetization. The horizontal axis represents the offset from the transmitter frequency. The transverse magnetization curve 40 has a flat excitation null 42 around the transmitter frequency.
The binomial pulse sequence shown in FIG. 3 has a total duration of approximately 5.4 ms at 2 Tesla. This is somewhat shorter than the gaussian pulse 2 of FIG. 1 but requires a high RF power because of the short "hard" pulses.
Suppression techniques generally extend the minimum TR that can be used and result in a reduction in the number of slices that can be imaged in a multi-slice sequence. They are also limited when short TR's are required since rapid, repeated, and incomplete, saturation of the fat frequency inevitably leads to a build up of coherent fat signal resulting in image artifacts.
Spectral-spatial excitation methods
Methods of Spectral-Spatial Excitation use a carefully designed RF modulation in the presence of an oscillating gradient. The result is a simultaneous selection along one spatial axis and the frequency spectrum. The use of Spectral-spatial excitation is disclosed in an article appearing in Magnetic Resonance in Medicine, entitled "Simultaneous Spatial and Spectral selective Excitation" by Craig H. Meyer et al. (Vol. 15, 1990, pp. 287-304), incorporated herein by reference.
FIG. 5 is a schematic graph illustrating imaging sequences designed for a prior art spectral-spatial excitation method. The horizontal axes of all the graphs of FIG. 5 represent time. The graph labeled RF represents the RF "fat free" imaging sequence. The vertical axis (not shown) of the graph labeled RF represents the amplitude of the RF. The graphs labeled G.sub.z, G.sub.x and G.sub.y represent the Gradient sequences wherein the horizontal axis represents time and the vertical axes(not shown) represent the magnetic field amplitude. The gradient sequence labeled G.sub.z is a modulated slice selection gradient. The gradient sequences labeled G.sub.x and G.sub.y are spiral readout gradients. Each of the gradients G.sub.z, G.sub.x and G.sub.y is shown as having a rephasing pulse at the far end of the gradient pulse trains.
The frequency of the modulated gradient is calculated so that, when centered on the water resonance, an excitation null occurs at the fat resonance. In this way only the water resonance is excited. This kind of pulse is usually incorporated directly into the imaging sequence since it is designed to select the desired slice profile only at the water frequency.
Reference is now made to FIG. 6 which is a schematic graph illustrating a prior art rapid gradient-echo pulse sequence using a spectral-spatial pulse with gaussian k-space varying along both k.sub.z and k.sub..omega. as disclosed by Meyer et al. The horizontal axes of all graphs represent time. The graphs labeled RFI and RFQ represent the real and the imaginary components of the RF "fat free" imaging sequence, respectively. In the particular example of FIG. 6, there is no imaginary component as illustrated by the flat trace of the curve labeled RFQ. The vertical axis (not shown) of the graphs labeled RFI and RFQ represent RF signal amplitude. The graphs labeled G.sub.z, G.sub.x and G.sub.y represent the Gradient sequences wherein the horizontal axis represents time and the vertical axes (not shown) represent the magnetic field amplitude. The gradient sequence labeled G.sub.z is a modulated slice selection gradient. The gradient sequences labeled G.sub.x is a readout gradient. The gradient sequence G.sub.y is a phase encoding gradient. This pulse sequence results in compact spatial and spectral slice profiles (not shown) which are gaussian in shape in the small-tip-angle regime.
Spectral-spatial techniques have the advantage of exciting only the chemical species of interest. Because of this, no sacrifice is necessary on the repetition time (TR).
However, spectral-spatial pulses are limited by gradient performance and are especially limited for low field applications where they are prohibitively long in duration. Additionally, careful optimization is required to ensure good spectral selection.